Health inspectors are looking into the possibility that the disease was originally transferred by employees at vaccine manufacturer Merial, based just a few miles from the farms.

Soil samples from the Merial site are being tested and the results are due on Thursday.

The easing of restrictions means that animals can be transported direct to slaughter and the collection of dead animals from farms can take place under licence.

Other movements of livestock, such as sending animals to market, are still banned.

Some restrictions had already been lifted in Scotland and Wales.

The Veterinary and Technical Director for the Meat Hygiene Service, Jane Downs, told the BBC's Today programme that, despite the easing of restrictions, it was still not business as usual.

"Animals can only move from outside the surveillance and protection zones and they have to go to specifically-approved abattoirs," she said.

"This is to minimise the risk of spread of infection should an animal be harbouring the disease and slaughterhouse operators have to comply with strict conditions, particularly focused on their bio-security."

'Remain vigilant'

National Farmers' Union president Peter Kendall said: "This is a measured and very necessary first step on the road towards getting the industry back to normal and maintaining supplies of home-produced meat to consumers."

The Health and Safety Executive has said there is a "strong probability" the outbreak began at the Pirbright research site, home to both Merial and the state-run Institute for Animal Health.

Both facilities had been using the strain of the virus, but the HSE did not specify which of the two was to blame.

It is understood, however, that investigations have discovered a link between problems with drainage and the possible actions, accidental or deliberate, of Merial employees who may use nearby allotments.

Merial said it had conducted "intensive internal investigations" and had "complete confidence" in its processes and procedures.

It confirmed one of its employees had accompanied investigators to an allotment but said there was "no evidence linking this member of staff to the outbreak".

Following criticism from some farmers and opposition parties, all footpaths within the 3km (1.8 mile) protection zone around the affected farms in Surrey have been closed.